Starting with version 10.7, OS X has dropped support for older PowerPC applications, including the original Avernum trilogy. However, there is no need for despair! Using an application called Wineskin, I have made Intel-native versions of the original games from the Windows versions. These are the demo versions. If you have purchased the games in the past, you can email Spiderweb Software support (firstname.lastname@example.org) with your registration code, and they will send you the activation code.
Because changing the window settings involves modifying the Wineskin configuration, I have made two versions, fullscreen and windowed.
The Exile trilogy has been released as freeware, but the Mac versions of the games will not run on any Intel Mac, as they are Classic Mac apps. I have made Wineskin Mac apps from the Windows versions of these as well. These are the full games and do not require registration codes.